The pragmatic solution is caulking or installing a molding.

The precise solution would be to scribe to the wall and ask a metal fabricator to cut your chimney precisely to those dimensions. I don't see how that would work for your existing chimney. So, most likely, they'd have to build a new one for you from scratch. That's going to be expensive, and there is a lot of potential for things to not quite work out.

The other thing you could try would be to slightly recess the hood into the sheetrock. That requires some careful work when you patch up the sheetrock afterwards. Do you have a level 5 finish on the walls? Can't quite tell from the photos. That would make it even harder, as imperfections would be extremely noticeable.

What brand? Zline sells stainless crown for many of their hoods.

Hello, curious what you ended up doing with the gap?

@fideligsay Just left it, and totally forgot about it. No regrets

Ah! The ultimate pragmatic solution.